1. Field of the Invention
The present invention relates to a printing plate processing apparatus that is capable of detecting center coordinates of register marks burned onto a printing plate based upon binary image data of the image captured by a CCD camera or the like, and a method of detecting the locations of the register marks.
2. Discussion of the Background
A method of detecting register marks performed by a conventional printing plate processing apparatus will be described with reference to FIGS. 9 to 11. FIG. 9 is a flow chart showing the steps of detecting the location of right and left register marks respectively burned onto a printing plate. FIGS. 10A and 10B respectively illustrate binary image data representing a white register mark in a black background, and a histogram curve obtained through the scanning of the binary image data of FIG. 10A and the calculation of a gray level histogram for each scanning line. FIGS. 11A and 11B respectively illustrate binary image data representing a black register mark in a white background, and a histogram curve obtained through the scanning of the binary image data of FIG. 11A and the calculation of a gray level histogram for each scanning line.
To detect the location of each register mark on the printing plate, the following conditions should be met:
1. An essential portion of the register mark is substantially equal in shape to a cruciform configuration.
2. The register mark has a negligible inclination with regard to the scanning direction.
3. Any contamination, patterns or other foreign matters with high contrast, which may confuse image recognition of the register mark, do not exist around the register mark.
4. The register mark has optical characteristics, enabling the register mark to be adequately captured within the image processing area of a corresponding CCD camera etc.
In the above requirements, the cruciform configuration includes T-configuration, and other forms which has at least two lines crossing each other to indicate the center coordinate of the register mark.
The image of each register mark captured by a corresponding CCD camera is displayed within the image processing area with vertical edges and horizontal edges, as illustrated in FIG. 10A. Based upon the binary image data of the register mark, a corner of a left edge and a lower edge is selected as the origin (0, 0), and the abscissa axis and the ordinate axis are respectively selected as X-axis and Y-axis. The binary image data of the register mark are scanned in the Y-direction for each pixel line with each pixel at a point (Y=0) as a starting point for the scan of each pixel line. The gray level histogram or white level of each pixel in each scanning line is then calculated (S1). Based upon the result of the calculation, a graph is made with the ordinate axis representing the white level and the abscissa axis as the representing the range of the image processing area (S2), thereby obtaining a histogram curve, as illustrated in FIG. 10B.
In FIG. 10B, A, B, C and D respectively represent the vertical line showing the maximum value of the white level, the maximum value of the white level, the vertical line showing the minimum value of the white level and the minimum value of the white level. The average value E of the sum of all the vertical lines are then calculated (S3).
The difference R between the maximum value B and the average value E, is compared with the difference L between the minimum value D and the average value E to determine which difference is larger than the other (S4). When L less than R, it is regarded as a white register mark in a black background (S5). When L greater than R, it is regarded as a black register mark in a white background (S6).
In case of L less than R, i.e., the white register mark in the black background, the slice level is obtained by the formula: (Maximum value B+Average value E)/2, and the comparison between the sum of the white level for each scanning line and the slice level proceeds in the right and left directions with an A-line as a starting line to determine G- and H-lines, where the sum of the white level of one scanning line becomes the same level as the slice level (S7). The line obtained by the formula: (G+H)/2 is then regarded as the center in the X-direction (S8).
When the G- and H-lines cannot be detected, the A-line is regarded as the center in the X-direction of the register mark (S9).
To obtain the center coordinate in the Y-direction of the white register mark, the scanning proceeds in the X-direction with each pixel at a point (X=0) as a starting point for the scan of each pixel line and with the corner of the left edge and the lower edge as the origin (0, 0), and the gray level histogram or white level of each pixel in each scanning line is then calculated in accordance with the flow of the step (S1) to the step (S9) to determine the center coordinate, in the same manner as the above.
In case of L greater than R, i.e., the black register mark in the white background, the slice level is obtained by the formula: (Minimum value D+Average value E)/2, and the comparison between the sum of the white level for each scanning line and the slice level proceeds in the right and left directions with a C-line as a starting line to determine I- and J-lines, where the sum of the white level of one scanning line becomes the same level as the slice level (S7). The line obtained by the formula (I+J)/2 is regarded as the center in the X-direction (S8).
When the I- and J-lines cannot be detected, the C-line is regarded as the center in the X-direction of the register mark (S9).
Then, to obtain the center coordinate in the Y-direction of the black register mark, the gray level histogram or white level of each pixel in each scanning line in the X-direction is calculated in accordance with the flow of the step (S1) to the step (S9), in the same manner as the above.
According to the method of detecting the location of the register mark as described above, since the detection of the register mark is detected based upon the gray level histogram of all the pixels within the image processing area with the corner of the left and lower edges as the origin (0, 0), it may take a longer period of time to detect the center coordinate.
When the register mark is positioned in the proximity of the printing image, the gray level histogram of the printing image is included in the gray level histogram of the X- and Y-directions obtained by the calculation of the binary image data within the image processing area. The result is the inclusion of a large error in the gray level histogram given through the calculation of the binary image data within the image-processing area, posing a problem of not being able to detect an exact location of the center coordinate of the register mark in prompt and exact manner.
In consideration of the above problem, it is an object of the present invention to provide a printing plate processing apparatus and a method of detecting the location of the register marks burned onto a printing plate, which are capable of detecting the exact locations of the center coordinates of the register marks, even if both the register mark and the printing image exist within the same image processing area.
To achieve the above objects, there is provided a method of detecting the locations of right and left register marks respectively burned onto right- and left-hand side portions of a printing plate based upon image data of images respectively captured within an image processing area with right and left vertical edges respectively extending in the Y-direction, and upper and lower horizontal edges extending in the X-direction. The method includes detecting the center coordinate in the X-direction of the right register mark based upon the image data obtained from substantially the entire portion of the image data processing area; detecting the center coordinate in the Y-direction of the right register mark based upon the image data obtained from a right-hand side portion of the image data processing area extending up to at least the center coordinate in the X-direction of the right register mark; detecting the center coordinate in the X-direction of the left register mark based upon the image data obtained from substantially the entire portion of the image data processing area; and detecting the center coordinate in the Y-direction of the left register mark based upon the image data obtained from a left-hand side portion of the image data processing area extending up to at least the center coordinate in the X-direction of the left register mark.
In accordance with the above method, the center coordinate of each of the register marks can be exactly detected, even if a print image is located in a right-hand side portion of the left register mark, or in a left-hand side portion of the right register mark.
The above method of the present invention may further includes determining if the right register mark is white or black by using a white level of the first several pixel lines from the right edge of the image processing area, and if the left register mark is white or black by using the white level of the first several pixel lines from the left edge of the image processing area.
In accordance with the above method, the register marks can be exactly recognized separately from a print image, even if the print image exists in the left-hand side of the image processing area including the right register mark, and in the right-hand side of the image processing area including the left register mark, since the register marks are not recognized based upon the sum of the all the pixel lines unlikely to the conventional method.
According to another aspect of the present invention, there is provided a printing plate processing apparatus for detecting the locations of right and left register marks respectively burned onto right and left-hand side portions of a printing plate based upon image data of images respectively captured in an image processing area with right and left vertical edges respectively extending in the Y-direction and upper and lower horizontal edges extending in the X-direction. The apparatus includes a camera for capturing images of the right and left register marks through the image processing area; an image processing means for converting the images of the right and left register marks into respective binary image data; a center coordinate detecting means for detecting the center coordinates in the X- and Y-directions of each of the right and left register marks based upon the respective binary image data within the image processing area; and a control means for controlling the center coordinate detecting means in such a manner as to detect the center coordinate in the X-direction of the right register mark based upon the image data obtained from substantially the entire portion of the image data processing area; detecting the center coordinate in the Y-direction of the left register mark based upon the image data obtained from a right-hand side portion of the image data processing area extending up to at least the center coordinate in the X-direction of the right register mark; detecting the center coordinate in the X-direction of the left register mark based upon the image data obtained from substantially the entire portion of the image data processing area; and detecting the center coordinate in the Y-direction of the left register mark based upon the image data obtained from a left-hand side portion of the image data processing area extending up to at least the center coordinate in the X-direction of the left register mark.
With the above method, the center coordinate of each of the register marks can be exactly detected, even if a print image is located in a right-hand side portion of the left register mark, or in a left-hand side portion of the right register mark.
The above apparatus of the present invention may further includes a comparator for determining if the right register mark is white or black by using a white level of the first several pixel lines from the right edge of the image processing area, and if the left register mark is white or black by using the white level of the first several pixel lines from the left edge of the image processing area.
Accordingly, the register marks can be exactly recognized separately from a print image, even if the print image exists in the left-hand side of the image processing area including the right register mark, and in the right-hand side of the image processing area including the left register mark, since the register marks are not recognized based upon the sum of the all the pixel lines unlikely to the conventional method.